1. Field of the Invention
The present invention relates to a switching amplifier containing a signal converter, several amplifier stages and a low-pass filter, in which arrangement the signal converter converts the input signal to be amplified into at least two pulse sequences and each amplifier stage has at least one switching stage having a transformer, the primary winding of which can be connected by means of at least one switching element, which is controlled by one of the pulse sequences, to a supply-voltage source and the secondary winding of which, which is electrically separated from the primary winding, is connected to the secondary windings of the other switching stages to form a series circuit and in which arrangement the low-pass filter is connected to the series circuit for converting the impulse sequences amplified in the switching stages into an amplified output signal.
2. Description of the Prior Art
Switching amplifiers of the above-noted type can be constructed for high output powers with high output voltage and have the good efficiency of class D amplifiers. Compared to the hitherto conventional linear amplifiers which are fed with high voltage and have special amplifier tubes with relatively short duty cycles, switching amplifiers with comparable characteristics can be simply operated with rectified mains voltage, contain commercially available long-life semiconductor switching elements and relatively small transformers.
A tested embodiment of a switching amplifier which can be used with particular advantage as a modulator for the transmitting tube of a broadcast transmitter is described in German Offenlegungsschrift No. 2,935,445. This switching amplifier contains several amplifier stages which are operated in parallel and of which each one has two switching stages and a signal converter which converts the input signal into two pulse sequences which are phase shifted by 180.degree. and have duration-modulated pulses. Each pulse sequence controls the switching elements in one switching stage of the amplifier stages, which enables the transformers to be prevented from saturating and 100 % modulation of the amplifier stages to be achieved. It is clear that in this operating mode, each switching stage and each transformer is excited once during each period of the pulse sequences.
In order to reduce the number of switching actions, it has also already been proposed (Swiss Application No. 992/81) to use a signal converter which converts the input signal into a plurality of pulse sequence pairs, the 180.degree. phase shifted pulse sequences of which consist of pulses having a maximum duration which is limited by the saturation time of the transformer and in which arrangement the number of pulse sequence pairs is modulated as a function of the analog input signal.
In addition, it has already been proposed (Swiss Application No. 116/82) to use the transformer as a push-pull transformer, which makes it possible to use the two pulse sequences or one pair of pulse sequences to excite the same transformer and thus to cut the total number of transformers in the switching amplifier by half.
As can be immediately seen by any expert, in a switching amplifier the losses in the amplifier and the distortion of the output signal are decisively determined by the construction and the characteristics of the transformer, independently of the type of modulation or of the excitation of the transformer.